Ferrofluids are colloidal dispersions (or suspensions) of magnetic nanoparticles in a liquid carrier that combine fluidic and magnetic properties to yield magnetically deformable liquids. Ferrofluids are well known and have many industrial applications including seals, coolants for loudspeakers and inks for printers.
More recently, ferrofluids have been employed for the fabrication of a new kind of deformable liquid mirror (Brousseau, D.; Borra, E. F.; Jean-Ruel, H.; Parent, J.; Ritcey, A. Opt. Express 2006, 14, 11486, and Laird, P.; Borra, E. F.; Bergamesco, R.; Gingras, J.; Truong, L.; Ritcey, A. Proc. SPIE 2004, 5490, 1493). Application of a current through a conducting liquid generates magnetic fields so that the liquid can respond to the force resulting from an externally applied magnetic field. By using a magnetic field, it is possible to shape the surface of ferrofluids and thus of the resulting liquid mirrors.
Since ferrofluids are not highly reflective, this application requires that they be coated with a reflective layer for mirror applications. Surface films of silver nanoparticles for coating ferrofluids are typically based on reflective liquid-like films and denoted as MELLFs (for Metal Liquid-Like Films) (Yogev, D.; Efrima, S.; J. Phys. Chem. 1988, 92, 5754).
To achieve a stable suspension of magnetic particles in the liquid carrier, stabilizing/dispersing agents are used to prevent particle aggregation and sedimentation. The choice of the stabilizing/dispersing agent depends on the nature of the liquid carrier in which the particles are dispersed. Ferrofluids composed of non-polar solvent, such as oils, usually include organic molecules containing relatively long alkyl chains, such as oleic acid, as the dispersing agent (E. Dubois, V. Cabuil, F. Boué and R. Perzynski, “Structural analogy between aqueous and oily magnetic fluids,” J. Chem. Phys., Vol. 111, No. 15, (1999)). When anchored to the particle surface, the organic tails prevent the aggregation of the magnetic particles by introducing steric repulsion.
In known preparations in polar media, particle aggregation is prevented by electrostatic stabilization achieved through the introduction of surface charges with methods analogous to those developed for aqueous ferrofluids. Typical procedures employ the surface adsorption of citrate (Dubois, E.; Cabuil, V.; Boué F.; Perzynski, R. J. Chem. Phys. 1999, 111, 7147) or hydroxide (Tourinho, F. A.; Franck, R.; Massart, R. J. Mater. Sci. 1990, 25, 3249) ions to produce negatively charged particles. As described below, ethylene glycol based ferrofluids stabilized in this way are not compatible with the reflective surface films of silver nanoparticles. The reflective layer gradually cracks and flocculates to the bottom of the container when deposited on a ferrofluid containing citrate-coated nanoparticles.
A relatively large number of organic ligands, including, for example, fatty acids (Dubois, E.; Cabuil, V.; Boué, F.; Perzynski, R. J. Chem. Phys 1999, 111, 7147), ionic surfactants (Massart, R.; Neveu, S.; Cabuil-Marchal, V.; Brossel, R.; Fruchart, J.-M.; Bouchami, T.; Roger. J.; Bee-Debras, A.; Pons, J-N.; Carpentier, M. Procédé d'obtention de supports magnétiques finement divisés par modification contrôlée de la surface de particules précurseurs magnétiques chargées et produits obtenus. French Patent 2,662,539, May 23, 1990) (Shafi, K. V. P. M.; Ulman, A.; Yan, X.; Yang N-L.; Estournès, C.; White, H.; Rafailovich, M. Langmuir 2001, 17, 5093), amines and alcohols (Boal, A. K.; Das, K.; Gray, M.; Rotello, V. Chem. Mater. 2002, 14, 2628.) have been investigated as stabilizing agents for magnetic nanoparticles. In all cases, however, these ligands were employed to enable particle dispersion in organic media. Particle stabilization in polar carrier liquids, such as water or ethylene glycol, has been achieved rather through the introduction of surface charges. In known preparations, particle aggregation is prevented by electrostatic stabilization employing the surface adsorption of citrate (Dubois, E.; Cabuil, V.; Boué F.; Perzynski, R. J. Chem. Phys. 1999, 111, 7147) or hydroxide (Tourinho, F. A.; Franck, R.; Massart, R. J. Mater. Sci. 1990, 25, 3249).ions to produce negatively charged particles. Bilayers of ionic surfactants have also been reported to provide electrostatic stabilization through the outer layer of charged head groups surrounding the particles (Maity, D.; Agrawal, D. C. J. Magn. Magn. Mater. 2007, 308, 46.)